(1) Field of the Invention
The present invention relates to a fixing device and in particular to a technology for managing a temperature of a heat generating rotational body.
(2) Description of the Related Art
In recent years, there has been proposed a fixing device that thermally fixes an unfixed toner image that has been formed on a sheet-like transfer material such as copy papers and OHP sheets and has a resistive heat layer included in a heat generating rotational body. In particular, such a fixing device has the resistive heat layer in a fixing belt, and performs fixing by directly supplying power to the resistive heat layer and causing the resistive heat layer to generate Joule heat (see Patent Literature 1). Such a fixing device has very high heat efficiency, since the fixing belt has low heat capacity and a distance from the resistive heat layer that is a heat source to the sheet-like transfer material that is a heated object is short. Therefore, it is possible to realize short warm-up and reduction in power consumption.
On the other hand, since the low heat capacity causes a temperature of the fixing belt to rise rapidly during heat generation, temperature management of the fixing belt is important. When a temperature of the fixing belt is out of a predetermined temperature range and in particular greater than a predetermined upper temperature limit, power supply to the resistive heat layer must be interrupted instantly. Otherwise, the temperature that is greater than the predetermined upper temperature limit is maintained or further increases in a short period, and accordingly a disadvantage such as breakdown of the apparatus might occur.
Conventionally, further temperature increase of the resistive heat layer has been prevented as follows. A surface temperature of the fixing belt is constantly measured, and when the measured surface temperature exceeds a predetermined value, a mechanical switch connected to a power supply circuit is turned off to interrupt power supply to the resistive heat layer.
However, high current is required to heat the resistive heat layer pertaining to the fixing belt, and the high current also flows through the mechanical switch. Accordingly, the high current might cause melting of the mechanical switch. During a normal operation, the mechanical switch is kept on. Accordingly, when melting occurs, the mechanical switch remains on. Therefore, there occurs a problem that, when a temperature of the fixing belt is out of the predetermined temperature range, it is impossible to turn the mechanical switch off to interrupt power supply to the resistive heat layer.    [Patent Literature 1] Japanese Patent Application Publication No. 2009-109997